$-4rs + 9rt - r + 8 = -7s + 10$ Solve for $r$.
Answer: Combine constant terms on the right. $-4rs + 9rt - r + {8} = -7s + {10}$ $-4rs + 9rt - r = -7s + {2}$ Notice that all the terms on the left-hand side of the equation have $r$ in them. $-4{r}s + 9{r}t - 1{r} = -7s + 2$ Factor out the $r$ ${r} \cdot \left( -4s + 9t - 1 \right) = -7s + 2$ Isolate the $r$ $r \cdot \left( -{4s + 9t - 1} \right) = -7s + 2$ $r = \dfrac{ -7s + 2 }{ -{4s + 9t - 1} }$ We can simplify this by multiplying the top and bottom by $-1$. $r= \dfrac{7s - 2}{4s - 9t + 1}$